1. Field of the Invention
The present invention relates to an image display apparatus and an image display method.
2. Related Art
In the conventional liquid crystal display apparatus, back light luminance control is conducted by dividing the screen into a plurality of regions with the object of expanding the display dynamic range and reducing the power dissipation. For example, according to JP-A 2005-258403 (KOKAI), back light luminance is controlled so as to make the back light luminance in each of regions obtained by division equal to at least the maximum display luminance of a video signal in the region.
When it is attempted to display an image expressing a light source in a dark background like, for example, a night view, however, it is necessary to set the back light luminance to a bright value in order to represent the brightness of the light source in a region expressing a bright light source in a dark background. If in this case the back light luminance is made bright, the dark background around the image of the light source is also displayed brightly. This is caused by the fact that it is in principle impossible to make the optical transmittance of liquid crystal small enough to display the dark background. On the other hand, if its peripheral region has only a dark background, then the back light is set to be dark and consequently the dark background is displayed sufficiently darkly. If a region expressing a bright source in a dark background and a region expressing only a dark background are adjacent to each other, the brightness of the displayed background differs between these regions and consequently unevenness occurs between the dark background regions. If the back light luminance difference between adjacent regions is small, it is possible to reduce the unevenness by correcting the optical transmittance of liquid crystal so as to make the optical transmittance of the liquid crystal relatively small in a region where the back light luminance is large and make the optical transmittance of the liquid crystal relatively large in a region where the back light luminance is small. However, since there is a limit in the black display capability of the liquid crystal as described above, it is difficult to remove this unevenness by only correction of the transmittance when the back light luminance difference between the adjacent regions is large.
For example, assume that it is attempted to display an image in which a light source having a relative luminance of 1.0 is expressed on a dark background having a relative luminance distributed in the range of 0.000002 to 0.001 when the minimum transmittance which can be implemented with liquid crystal is 0.002. In this case, since it is necessary that the relative luminance is 1.0 in the region where the light source is expressed, it is desirable to set the relative luminance of the back light equal to 1.0. Therefore, the range of the relative luminance which can be represented in this region is a range of 0.002 to 1.0 obtained by multiplying the relative luminance of the back light by the transmittance of the liquid crystal. On the other hand, in a region where the light source is not expressed, it is desirable to set the relative luminance of the back light equal to 0.001 in order to make it possible to represent the relative luminance 0.000002 to 0.001 sufficiently. Therefore, the range of the relative luminance which can be represented in this region is a range of 0.000002 to 0.001 obtained by multiplying the relative luminance of the back light by the transmittance of the liquid crystal. As a result, a dissimilarity occurs in the range of the relative luminance which can be represented between the two regions. In principle, it is impossible to compensate the luminance difference between the two regions. Therefore, unevenness occurs between the two regions.